Stacks

A stack is a Last-In, First-Out (LIFO) data structure. The last element added is the first one removed — like a stack of plates.

Core Operations

Stack in C

var stack = new Stack<string>();

stack.Push("First");
stack.Push("Second");
stack.Push("Third");

Console.WriteLine(stack.Peek());  // "Third"
Console.WriteLine(stack.Pop());   // "Third"
Console.WriteLine(stack.Pop());   // "Second"
Console.WriteLine(stack.Count);   // 1

Example: Balanced Parentheses Checker

A classic stack problem — check if brackets in a string are balanced:

public static bool IsBalanced(string input)
{
    var stack = new Stack<char>();
    var pairs = new Dictionary<char, char>
    {
        { ')', '(' }, { ']', '[' }, { '}', '{' }
    };

    foreach (char ch in input)
    {
        if (ch is '(' or '[' or '{')
        {
            stack.Push(ch);
        }
        else if (pairs.ContainsKey(ch))
        {
            if (stack.Count == 0 || stack.Pop() != pairs[ch])
                return false;
        }
    }

    return stack.Count == 0;
}

// Usage
Console.WriteLine(IsBalanced("{[()]}")); // True
Console.WriteLine(IsBalanced("{[(])}"));  // False
Console.WriteLine(IsBalanced("((())"));   // False

Common Use Cases

• Undo/Redo — push actions onto a stack, pop to undo
• Expression parsing — compilers use stacks to evaluate expressions
• Backtracking — DFS, maze solving, browser history
• Call stack — method calls in your program use a stack internally